Silicon On Insulator (SOI) technology is becoming increasingly important for high performance thin film transistors, solar cells, etc. SOI wafers consist of a thin layer of substantially single crystal silicon (generally less than one micron) on an insulating material.
Various structures and various ways of obtaining such wafers are known. Typically, used structures are formed with a thin film of single crystalline silicon 0.01-2 μm in thickness bonded to another silicon wafer with an oxide insulator layer in between.
Because of its rather high thickness, in particular as compared to the other parts, a major fraction of the cost of such structures has been the cost of the silicon substrate that supports the oxide layer, topped by the thin silicon layer. Thus, to lower the cost of SOI structures, the use of support substrate made of materials less expensive than silicon has been tried, in particular glass or glass-ceramics.
SOI structures using such glass-based substrates are called SiOG structures, as already mentioned. Processes for providing a SiOG structure are, for example, described by U.S. Pat. No. 7,176,528. Such a process is represented by FIG. 1. A source substrate 1, generally made of silicon, is oxidized and implanted with ionic species 6 such as hydrogen. The implantation leads to the creation of a buried, weakened zone 2. Further, the source substrate 1 is bonded with a glass-based support substrate 3 and then separated by splitting the source substrate 1 at a depth corresponding to the penetration depth of the implanted species 6 (the separation zone 2). In this way, a SiOG structure containing the original glass-based support substrate 3 and a layer 4 originating from the source substrate 1, and a remaining delaminated substrate being a part of the former source substrate 1 are produced.
However, it is not a simple matter to replace a traditional silicon support substrate with a glass-based support substrate. One potential concern with SiOG is that the glass-based support substrate 3 contains metal (in particular alkalis) and other components that may be harmful to the silicon or other semiconductor layer 4. Therefore, a barrier layer may be required between the glass-based support substrate 3 and the silicon layer 4 in the SiOG. In some cases, this barrier layer facilitates the bonding of the silicon layer 4 to the glass-based support substrate 3 by making the bonding surface of the silicon layer 4 hydrophilic. In this regard, one SiO2 layer may be used to obtain hydrophilic surface conditions between the glass-based support substrate 3 and the silicon layer 4.
A native SiO2 layer may be formed on the silicon source substrate 1 when it is exposed to the atmosphere prior to bonding. Additionally, the anodic bonding process produces “in situ” SiO2 layer between the silicon source substrate 1 and the glass-based support substrate 3. If desired, one SiO2 layer may be actively deposited or grown on the source substrate 1 prior to bonding. Another type of a barrier layer provided by the anodic bonding process disclosed in U.S. Pat. No. 7,176,528 is a modified layer of glass in the glass-based support substrate adjacent to the silicon layer with a reduced level of ions. Anodic bonding substantially removes alkali and alkali earth glass constituents and other positive modifier ions that are harmful for silicon from about 100 nm thick region in the surface of glass adjoining the bond interface.
Glass material differs also on some other physical properties when compared to traditional silicon support as, for example, stiffness and this limited compatibility with silicon has an impact on the surface texture of the transferred layer 4 of a SiOG structure.
Indeed, splitting the source substrate at the separation zone generates particularly numerous and deep surface irregularities, as represented in FIG. 2. It combines plateaus 41, which surface presents microroughnesses 42, the plateaus being encircled by pits 43 named “canyons.” Such surface irregularities have to be eliminated, and a chemical-mechanical polishing (CMP) is classically performed to that effect. However, such polishing is long and expensive, in particular if the plateaus present a high-density of microroughnesses 42. Moreover, the more the canyons 43 are deep, the more the layer of material to be removed by polishing is thick. An important amount of high-grade silicon is therefore wasted. According to FIG. 3, which represents the profile of a transversal section, the band to be removed can have a thickness extending to 10 nm.
There is consequently a need for a solution for reducing the depth and the density of canyons, and for limiting the microroughnesses.